Anti-design is the design of a purpose to justify a product, rather than the product to satisfy a purpose.
A regression is some aspect or configuration of a system that used to work but now does not.
This is an artificial superordinating category for pathologies in system design that relate to time.
Un-design is a side effect of process. If design is the rendering of intent, un-design lacks any and all intent. A particular identifiable configuration of a system can be said to be "un-designed" if it lies at the intersection of—or is the result of the interaction between—two or more designed (or even themselves un-designed) entities. Contrast "anti-design".
A vestige is some aspect or configuration of a system that used to serve a purpose but no longer does. It differs from a Chesterton's fence insofar as its superfluity is known.